Simplify cruft test of license by using given/when structure

Simplify the cruft test by using a case like structure

Signed-off-by: Niels Thykier <niels@thykier.net>
Detect empty gfdl statement about invariant sections

gfdl requests to explicitly mark that you have not invariant section
by writing text like this:

  "with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts."

Some author instead use empty text. Detect this usage.

Signed-off-by: Niels Thykier <niels@thykier.net>
Avoid false positive with texi

Some texi file use gfdl with @c for comment instead of space.
Avoid false positive by allowing @c

Signed-off-by: Niels Thykier <niels@thykier.net>
Simplify grouping by using ?'name'

?'name' is better for documentation than $1

Signed-off-by: Niels Thykier <niels@thykier.net>
Avoid a false positive with tex comments

Tex comments % are allowed in the gfdl.

Signed-off-by: Niels Thykier <niels@thykier.net>
Be more liberal on space matching for gfdl detection

Replace \s? by \s* (or exactly $s? by $s*)

Signed-off-by: Niels Thykier <niels@thykier.net>